A central question in neuroscience is how individual neurons connect with one another during development. However, mechanisms of synapse formation are still poorly understood. Many of the proteins and organelles present at mature synapses have been identified, but how these components arrive at newly forming synapses is not known. Two recent studies, including one from this laboratory, have implicated dense core vesicles (DCVs) in the transport of proteins to the presynaptic active zone during synapse formation. DCVs are a major type of secretory vesicle involved in signaling between neurons. They secrete neuropeptides, hormones, neurotrophins and certain neurotransmitters. These studies now suggest an additional role for DCVs in the transport of presynaptic proteins to newly forming synapses. The goals of this proposal are to determine whether the DCVs involved in protein transport differ from those involved in secretion, and to understand the precise role of DCVs in synapse formation. In the first two specific aims, we will use immunostaining and optical imaging to describe the properties of DCVs involved in protein transport. In the third specific aim, we will use antisense RNAs to reduce or eliminate DCVs in developing neurons, then examine how synapse formation is affected.